This invention relates to electromagnetic flowmeters for monitoring the rate of flow of an electrically conductive fluid or slurry and is more particularly, but not exclusively, concerned with such flowmeters utilising a system of excitation in which the field coils are periodically pulsed at low frequency.
In an electromagnetic flowmeter the fluid or slurry whose flow rate is to be measured is conducted along a flow tube within which a magnetic field is established by an electromagnet transversely to the axis of the tube. By Faraday's law of electromagnetic induction, an e.m.f. is thereby induced across the flowing fluid or slurry transversely to the magnetic field, the magnitude of the e.m.f. being proportional to the rate of flow of the fluid or slurry. This e.m.f. is detected by appropriately positioned measuring electrodes, and the signal thus detected is amplified and converted into direct current form before being supplied to an indicator or recorder.
Until recently most electromagnetic flowmeters utilised a.c. excitation of the field coils. This avoids the problem of polarization of the electrodes obtained when d.c. excitation is used. However, the use of a.c. excitation has distinct drawbacks in that various interference voltages unrelated to flow rate are induced in the measuring electrodes. This has led to flowmeters being proposed in which the field coils are excited by low frequency d.c. pulses. However, more particularly where the flowmeter is being used to measure the flow rate of a slurry, it has been found that the noise component due to disturbances at the electrode/slurry interface lies within the same frequency range as the flow-induced measurement signal, and this may severely limit the accuracy of measurement. It has been proposed in U.S. Pat. No. 4,296,636 to minimize the noise component by forming the electrodes from an extremely hard metal or metal alloy. However, the results obtained with such electrodes have been found to be unsatisfactory.
It is an object of the invention to provide an electromagnetic flowmeter which exhibits high measurement accuracy even when it is used to measure the flow rate of a slurry.